The present invention relates to telecommunication switching systems and more particularly to switching systems for use in digital transmission systems including so-called synchronous digital hierarchy (SDH) systems.
In such systems digital data is switched by so-called add-drop multiplexers (ADMUX) from a main or core network by a "master" ADMUX into sub-loops each containing a plurality of "slave" ADMUXs each of which is adapted to switch data in the sub-loop into a so-called tributary.
Because of the magnitude/bandwidth requirements of the data flow through the master ADMUX compared with that through the slave ADMUXs the master ADMUX needs to have a greater capacity than the slave ADMUXs. In contrast in order to minimise the manufacturing costs it is preferable for identical standard ADMUXs to be used for both the master ADMUX and the slave ADMUX.
As a result the capacity of all the ADMUXs in the loop is dictated by the relatively high capacity required for the master ADMUX. This means that the slave ADMUXs have a higher capacity than is actually required and as a result the limiting factor on the overall capacity of the loop/ring lies in its interconnections between the ADMUXs. Furthermore, because of the excess capacity when compared with the links between them of the slave ADMUXs there could be circumstances in which the volume/bandwidth data being switched out of the loop by a particular slave ADMUX or ADMUXs is so great that there is no capacity left in the links to enable data to be fed to a slave ADMUX which is downstream of that particular slave ADMUX. It is with this problem that the present invention is concerned.
The present invention is also concerned with adapting an ADMUX so that it can interface with a higher data rate system than that for which it was originally designed.
In the above mentioned synchronous digital hierarchy transmission systems data is transmitted in the form of so-called synchronous transport modules (STM) and these modules are multiplexed at a certain data rate. More particularly in the art reference is made to STM-1 which refers to a synchronous transport module (STM) multiplexing at level 1 which is a signal rate of 155.52 Mbit/s, conforming to ITU (previously CCITT) specification G708. Similarly STM-4 refers to a synchronous transport module multiplexing at level 4 at a signal rate of 622.08 Mbit/s consisting of 4 byte interleaved signals operating at 155.52 Mbit/s and conforming to ITU specification G708. Furthermore STM-16 refers to a synchronous transport module multiplexing at level 16 and having a signal rate of 2488.32 Mbit/s consisting of 16 byte interleaved signals operating at 155.52 Mbit/s and conforming to ITU specification G708.
The present invention is therefore also concerned with adapting an ADMUX which was originally designed for use in an STM-1 system so that it can also be used in an STM-4 system and also for adapting an STM-4 ADMUX which was originally designed for use in an STM-4 system so that it can then be used in an STM-16 system.
JP-A-5160789 discloses an add-drop multiplexer for use in a synchronous digital hierarchy telecommunications system provided with a by-pass, whereby it can be made to interface with a higher bit rate transmission system in that the bit rate on the input signal is higher than the signal applied to the add-drop circuit. The use of ADMUXs in a sub-ring of a telecommunications network is disclosed in Telcom Report, Vol. 15, No. 5 at pages 194-197.